This invention relates to a cutting tool coated with diamond. More particularly, it relates to a throw-away cutting tool coated with a film of diamond and which is used as a cutting bit or insert, a milling cutter, an end mill or a drill.
[Definition]
The term xe2x80x9cdiamondxe2x80x9d or xe2x80x9cdiamond filmxe2x80x9d used herein denotes not only pure crystalline diamond but what is synthesized as a diamond film, i.e., embracing diamond-like carbon, graphite or amorphous carbon, or a mixture thereof. Such diamond may be sometimes called diamond-or-the-like or pseudo-diamond.
If diamond is applied in a film shape on a cemented carbide substrate, a cutting edge is rounded due to the presence of the diamond film. The rounding becomes more pronounced the thicker the diamond film, with the result that the tool is lowered in cutting performance. Moreover, since irregularities due to the diamond crystals are present on the as-coated diamond film surface, the cutting chips are hardly removed to produce a large cutting resistance. In addition, welding tends to be produced to affect the durability of the cutting tool.
The routine practice for coping with this problem has been to grind the diamond film surface, after diamond coating, to sharpen the cutting edge, or to smooth the film surface to prevent welding.
For sharpening the cutting edge of the cutting tool, there is disclosed in JP Patent Kokai JP-A-3-67602 a technique of polishing both the rake surface and the flank surface, following diamond coating, to sharpen the cutting edge. On the other hand, there is disclosed in JP Patent Kokai JP-A-4-201102 a technique of polishing the rake surface smooth following diamond coating, and of polishing the diamond film off from the flank surface to sharpen the cutting edge.
Among known techniques used for polishing natural or artificial single crystal diamond, there are techniques such as co-grinding with a diamond wheel having diamond abrasive grains or diamond particles embedded therein and scarfing polishing employing a cast iron plate. However, if, in these methods, the co-grinding pressure is raised, or the tool is contacted with a diamond wheel rotating at an elevated speed, the coated diamond film tends to be peeled off.
Thus, in the JP Patent Kokai JP-A-62-41800, JP Patent Kokai JP-A-63-144940 and in the JP Patent Kokai JP-A-63-57160, such a method is proposed in which, for improving the polishing efficiency, the diamond surface of the substrate is contacted in a non-oxidizing atmosphere with a heated metal surface to graphitize the diamond on the film surface to remove the graphitized diamond.
As other smoothing methods, there is disclosed in JP Patent Kokai JP-A-4-331800 a method of sputtering the diamond film surface with an ion beam. There has also been reported in Yoshikawa et al., Processing or CVD Diamond Films by YAG Laser: Journal of Precision Engineering, 55, 12(1990) 2256, a method of machining the diamond film by the laser light.
There is also proposed, in JP Patent Kokai JP-A-3-190605, a method of polishing a portion of a diamond film on the substrate of the cutting tool which is formed on the cutting edge and in a portion extending from the cutting edge to a portion of the flank surface, using a brush carrying deposited polishing abrasive grains.
In connection with the shape or the structure of stepped portions on the rake surface of the tool cutting, there is shown in JP Patent Kokai JP-A-7-60509 such a shape or structure in which triangular-shaped steps (convexed portions) are provided at the four corners of the square-shaped substrate, cemented carbide films, such as diamond films, are applied to these portions, and only the cemented carbide film portions at the corner portions are polished to a smooth finish as compared with the surface roughness encountered at the time of film deposition (see FIGS. 7 and 10).
However, in the course of the investigations toward the present invention the following problems have been encountered. Namely, the problems of the above-described conventional techniques are hereinafter scrutinized.
The techniques proposed in the above-mentioned JP Patent Kokai JP-A-3-67602 or in the JP Patent Kokai JP-A-4-201102 suffer a problem that the polishing operation is time-consuming because the entire rake surface needs to be polished. On the other hand, if the diamond film on the flank surface is polished off, the state of an edge portion is such that the diamond film and the boundary surface of the substrate are exposed to the exterior, so that the tool cannot withstand heavy duty cutting such as on aluminum alloys and hence the film tends to be peeled off.
The method of graphitizing and removing diamond, as disclosed in the above-mentioned JP Patent Kokai JP-A-62-41800, suffers problems that the method requires heating to elevated temperatures, and is necessarily of the batch type because the processing is carried out under the non-oxidizing atmosphere, and that a special equipment is required for assuring safety if hydrogen is used as an atmosphere, so that the method is not suitable for industrial production.
On the other hand, the technique of sputtering or machining of the diamond film surface by an ion beam or the laser suffers a problem that the apparatus is complex in structure and low in mass producibility such that the technique is not suitable for industrial production.
With the method of polishing the cutting edge of the diamond film using a brush carrying deposited grinding grains, as proposed in JP Patent Kokai JP-A-3-190605, the major portion of the irregularities ascribable to the inherent shape of the diamond grains in the vicinity of the edge are removed on polishing, so that it is possible to prevent the lowering of the weld as well as to prevent the work surface from becoming roughed due to transcription of the irregularities ascribable to the inherent shape of the diamond grains. However, since the polishing is carried out by a brush, a rounded tool cutting portion cannot be sufficiently removed due to dulling by polishing, with the result that sharp edges usable for finishing machining cannot be obtained. There is also a problem that technical difficulties are encountered in polishing only the vicinity of the edge portions by a simple well-known technique other than polishing with a brush. For example it is difficult to bring the diamond wheel into partial grinding contact with the work to high precision, such that the film coating tends to be peeled off.
Moreover, since the diamond film is inferior in general in tight adhesion to the substrate, in particular to the cemented carbide substrate, researches are proceeding for improving the adhesion of the diamond film. The present inventors have proposed a cutting tool with a coating of diamond which, by surface-processing the substrate by heat treatment (JP Patent Kokai JP-A-7-90321, corresponding to U.S. Pat. No. 5,858,480 and U.S. Pat. No. 5,725,932 and EP 0627498A) or by electrolytic etching (JP Patent Kokai JP-A-10-310494, corresponding to U.S. patent application Ser. No. 08/977,972, pending, and EP 0864688A), is endowed with sufficient adhesion to withstand heavy-duty cutting.
However, there are occasions where the substrate becomes deformed by surface processing of the substrate to affect the polishing operation on the coated diamond film. Specifically, there are occasions where a mid portion of the cutting surface of the substrate becomes convexed (warped) upwards due to heat treatment such that the mid portion of the cutting surface becomes higher than the cutting edge by 5 to 30 xcexcm. In the case of electrolytic etching, the speed of electrolytic etching at the cutting edge portion becomes faster than at other portions due to a higher current density, thus increasing the amount of the substrate removed from the cutting edge portion. This occasionally gives the shape of the tool in which the cutting edge portion is lower in height than the mid portion of the rake surface by to 5 to 15 xcexcm.
In any of the above-described techniques, the mid portion of the rake surface of the substrate becomes higher in height than the cutting edge portion, so that, if the polishing is performed after coating the diamond film for sharpening the cutting edge, the convexed mid portion of the rake surface is initially contacted with the polishing wheel, however, the cutting edge, which is to be polished, is hardly polished.
If the diamond film is polished, using a metallic material heated to raise the polishing speed, as disclosed in the aforementioned JP Patent Kokai JP-A-62-41800, prolonged machining is required for polishing up to the cutting edge sufficiently. Also, if the substrate has warping, the mid portion of the rake surface needs to be polished in addition to the portion that inherently needs to be polished, thus lowering the efficiency. Also, depending on the degree of the warping and the thickness of the diamond coating film, the diamond film is removed excessively to expose the substrate to the exterior.
In a cutting tool in which the triangular-shaped steps (convex portions) are provided at respective corners of the square-shaped rake surface, as disclosed in JP Patent Kokai JP-A-7-60509, as shown in FIG. 10, there is a risk of the polishing quantity being increased to lower the polishing efficiency if, when the rake surface undergoes convexed warping, account is not taken of the height of the steps of the rake surface, diagonal lengths of the entire rake surface or of a width 105 of the step of the rake surface in the diagonal direction.
It is an object of the present invention to provide a cutting tool for diamond in which the cutting edge can be sharpened by polishing for only a short time and which can be conveniently used in particular as a tool for the finishing machining.
A substrate according to the present invention has its portion raised at a pre-set width along a cutting edge on a rake surface and/or on a flank surface. If diamond is applied as a film on a substrate having this shape, the film of diamond (may be termed as xe2x80x9cpseudo-diamond) in the vicinity of the cutting edge as an effective cutting portion has a height larger than the remaining portions. The result is that, if the vicinity of the cutting edge is polished for sharpening the cutting edge or smoothing the effective cutting edge face, it becomes possible to preferentially polish the film of diamond in the vicinity of the cutting edge, without first polishing the film of diamond formed centrally of the rake surface and/or the flank surface, thus realizing the maximum effect (i.e., sharpening of the cutting edge and prevention of welded cutting chips) with the necessary minimum polishing (with a decreased removal by polishing). The result is that the sharpening of the cutting edge and smoothing of the effective rake surface can be easily realized in short time by polishing by scarfing (polishing over a wide area) suited to industrial production.
The substrate according to the present invention is useful as a substrate subjected to pre-processing (surface modification), such as heat treatment, for improving tight adhesion between the film of diamond with the substrate. The reason is as follows: There are occasions where this heat treatment deforms the substrate or causes warping such that the rake surface and/or the flank surface is raised at its center portion. If, due to this warping, the film of diamond at the center of the rake surface and/or the flank surface is higher in level than the diamond film in the vicinity of the cutting edge, it is necessary to polish the diamond film at the mid portion excessively in an amount corresponding to the warp. Since the diamond is the hardest of various materials, this difference in the polishing amount gives rise to an appreciably prolonged period of time for machining. According to the present invention, the removal by polishing for this portion that need not be polished can be decreased significantly or even reduced to zero.